1. Field of the Invention
Embodiments of the invention relate to a powder mixture for manufacture of a battery electrode, a respective battery electrode, and a method of manufacturing of an active material powder mixture for the production of battery electrodes, particularly for lithium batteries. Embodiments of the invention further relate to the method for manufacturing of a respective battery electrode.
2. Description of the Related Art
Batteries are sources for electrical energy which are commonly used. Important elements of a battery are the negative electrode, typically called the anode, and the positive electrode, typically called the cathode. Usually, the anode comprises active material that can be oxidized and the cathode comprises active material that can be reduced.
One well-known method for manufacturing of an electrode (anode or cathode) uses powder mixtures. For example, battery electrodes are produced by wet coating methods. In these methods, the powder is dispersed together with a binder in a solvent. The solvent is selected for this purpose so that it wets the components of the powder sufficiently and simultaneously it has a sufficiently high solubility for the binder. Aqueous dispersions or dispersions based on organic solvents (e.g., alcohols, ketones, amines, amides, ethers) are used for this purpose. These dispersions are applied to an electrically conductive carrier (e.g., metals, conductively coated polymers) and the solvent is removed by drying.
The disadvantages of this method are manifold. The production of the dispersions is time-consuming and costly and the useful lives of the dispersions are frequently limited by coagulation or sedimentation. Furthermore, secondary reactions occur due to the decomposition of the solvent(s), possibly also due to secondary reactions with the dispersed substances or with the dissolved binders. These lower the useful lives of the coating solutions. Disadvantages arise both with aqueous coating solutions and also with organic solvents. Aqueous coating solutions are nontoxic, but may only be dried with difficulty. Long drying times or high drying temperatures are necessary. Moisture-sensitive materials may not be coated by aqueous solvents. Organic solvents typically require a high technical outlay (explosion protection, fire protection, solvent reclamation/solvent combustion). The requirements for labor protection (toxicity, annoyance due to bad smells) are typically high. Quantitative reclamation is typically not possible. The production of homogeneous coatings is technically demanding and costly because of the rheological properties of the dispersions and the drying of such coatings produces high energy costs and is time-consuming.
Above all in lithium batteries, the selection of the binders stable under the operating conditions of the battery is limited. Often, only fluorinated polymers are usable. These are frequently only soluble in special, expensive organic solvents having high boiling points. The solvents increase the porosity of the coating during the drying process. To increase the energy density of the coating, additional work steps are therefore necessary after the drying to compact the electrodes (pressing, calendering).
Further methods known from the prior art are pressing methods. In these methods, powder mixtures are compressed to form tablets, rings, or cups (pressed parts). The powder mixtures may also be processed to form strands or films by extrusion. The electrical discharge of the current occurs in this case by laminating on a metallic current collector, possibly with the aid of an electrically conductive adhesive. The contact may also be produced by a simple press contact. The discharge is frequently ensured by a press contact with the housing of the battery or by a nail which is driven into the pressed part.
These methods also have disadvantages. The contact areas of the pressed parts for the press contacting are typically small, the carrying capacity of the batteries is limited. Furthermore, disadvantages have been established if a homogeneous material bond is not produced between the current collector and the housing (e.g., by welding). The transition resistance of the press contact may then rise in the course of the battery life due to the formation of cover layers, due to gas development, or due to corrosion. A rise of the transition resistance of press contact has also been established when the pressure which acts on the press contact is reduced in the course of the battery life. The causes of this could, for example, be an expansion of the housing due to swelling of the battery materials or due to gas development.
In hermetically sealed batteries, in which no polymer seals could be used for sealing and insulating housing parts, only one electrode could be electrically connected by a press contact. The other electrode must be electrically connected to the battery housing using a bushing insulated from the battery housing (e.g., a glass-metal bushing). A connection of the electrode to a current collector is required for this purpose.
A further grave disadvantage is the low mechanical stability of the pressed parts. Because of this disadvantage, there must be a sufficiently high wall thickness according to the prior art. This results in a low electrical carrying capacity of the battery. For mechanical stabilization, the pressing method may be performed directly in the battery housing (e.g., with alkali manganese batteries). This requires sufficient material thicknesses of the housing and is bound to suitable housing geometries (cylindrical housing). It has also been shown that laminating on a current collector represents an additional time-consuming and costly work step.
In many cases powdered materials and/or mixtures of powdered materials are used to produce battery electrodes. These are coated on an electrically conductive material, which is used as the current collector. The powdered materials are therefore preferred because they form porous structures having a large surface in relation to the ion-conducting electrolytes. A high carrying capacity of the battery electrode per unit area is thus achieved.
In particular for thin electrodes e.g. for high-power applications in implantable Cardioverter Defibrillators with typically electrode thicknesses below 0.8 mm fine powders of an active material, binder or, if applicable, additive have to be used. Additionally, the powders of the components of the electrode must be dosed exactly, in order to reach an exact composition of the electrode material.
Fine powders or mixtures of fine powders show a limited flowability, in particular if materials with low density are used (for example carbon black or graphite, which are often used as additives). This limited flowability leads to difficulties in powder dosing, in particular in an automated production process. This property is particularly a disadvantage if the powders or powder mixtures are used to produce low bulk heights with uniform layer thickness.
Powder is a disperse system which consists of a plurality of single particles. A bulk powder mixture is a homogeneous mixture of powders (primary particle system) without stable interparticular connection.
Different methods for manufacturing of electrode powder mixture are known. Document DE 10 2007 034 178 A discloses a method for manufacturing of electrodes using suspensions comprising active nano material. The documents U.S. Pat. No. 7,527,895 B2 and U.S. Pat. No. 6,946,220 describe the production of high power electrodes for lithium batteries by wet coating.
Therein, the additives (for example black carbon or graphite) are pre-mixed with the active material (for example LiNiCoAlO2, MnO2, FeS2, AgxV2Oy) and are then suspended in a binder solution. Within the binder solution a polymer solvent (for example PvdF, PTFE) adsorbs to the surface of the active material particle forming a binder coating. The coating promotes the adhesion of the additive particles.
Such suspensions are then applied onto current collectors (for example a grid or a foil) using a wet coating apparatus. The coated current collectors are then dried to the necessary residual moisture for lithium batteries on a drying band-conveyer. The electrode packet (anode or cathode) is then produced using a winding or stacking technology.
From document U.S. Pat. No. 5,571,637 it is known that a wet active material (paste) is compressed in order to get electrode pellets. The pellets are subsequently dried. Additionally, dry-pressing of active material powder is described in order to produce electrodes.
During wet coating of solvent containing suspensions according to the state of the art methods a drip time of the coating solutions has to be considered. Another disadvantage of these state of the art methods is that solutions sediment or exhibit a limited handling duration. The handling duration is getting the shorter, the more reactive the used active material with the binder, the solvent or the additive is.